The present invention relates to a method and an apparatus for optically checking any defect in the surfaces of magnetic disks, and more particularly to an optical checking method and apparatus for defects in magnetic disks suitable for detecting any dent in the surfaces of and foreign matters sticking to both faces of conventional magnetic disks.
Known apparatuses for checking any defect in the surfaces of magnetic disks which are magnetic media for continuous recording are described in, for instance, JP-A No. 2002-257742 and JP-A No. 2008-268189. JP-A No. 2002-257742 discloses a configuration in which a rotating magnetic disk is irradiated with a laser beam in a slanted direction to have a specular reflection from the disk surface form an image on a detector provided with multiple light receiving elements and its detection signals are processed to detect defects.
JP-A No. 2008-268189 discloses a checking apparatus which irradiates a rotating magnetic disk with a laser beam, detects a specular reflection and scattered light from the disk surface, and detects and classifies defects on the disk surface by processing respective detection signals of the specular reflection and the scattered light.
On the other hand, though it is known that the laser beam that irradiates the magnetic disk has a Gaussian distribution of power in the cross section of its beam, it is described in JP-A No. Hei 6-347418 that observation at a uniform illuminating intensity is achieved by shifting a laser beam having a Gaussian distribution at intervals substantially equal to the radius of the laser flux along multiple sections of the object of observation thereby to superpose the flux over the multiple sections.
Along with the increase in recording capacities of magnetic disks, recording on magnetic disk is becoming higher in density and the defects to be detected are becoming smaller in size.
FIG. 10A and FIG. 10B show external views of a sample (magnetic disk) 100 to be checked according to the invention. FIG. 10A shows the magnetic disk 100, and FIG. 10B schematically shows an expanded partial view of a section thereof. The magnetic disk 100 has a structure in which a magnetic film 1012 (actually it is structured of a multi-layered film formed over a foundation layer) is uniformly formed over both faces of a substrate 1011 and the surfaces are covered by protective films 1013. Therefore, in an ideal state in which the surfaces have no defect at all, when the surface of the magnetic disk 100 is irradiated with a laser beam in a slanted direction, reflected beams having a distribution matching that of luminous energy in the section of the irradiating laser are detected from the surface of the magnetic disk 100.
On the other hand, the distribution 1101 of the luminous energy in the section at a right angle to the optical axis of the laser beam irradiating the surface of the magnetic disk 100 has a so-called Gaussian distribution pattern in which, as shown in FIG. 11, the energy is the strongest at the center (center of the optical axis) and weakens with the distance away from the center.
Usually, when the magnetic disk 100 is to be checked, the magnetic disk 100 is turned and any defect is extracted by detecting with a detector the reflected beams from the adjacent bands in the radial direction of the magnetic disk surface in each turn. In this process, each position of irradiation of the magnetic disk surface with the laser beam is caused to partially overlap with the immediately preceding position; thus the irradiated area is kept greater than the coverage of detection by the detector.
As a consequence, checking at a given point of time is influenced by reflected beams (scattered light) from a defect which existed in the irradiated area one turn before but is absent in the irradiated area at the current point of time, and an error in the position of detection is invited when greater accuracy of detecting the defective position is desired.